The invention relates to a method of distributing a key for enciphering an unenciphered or plaintext message and for deciphering the enciphered message.
A public key distribution method used in a public key cryptosystem as a well-known key distribution method is disclosed in a paper entitled "New Directions in Cryptography" by W. Diffie and M. E. Hellman, published in the IEEE Transactions on Information Theory, Vol. IT-22, No. 6, pp. 644 to 654, November issue, 1976. The key distribution method disclosed in the paper memorizes public information for each of conversers. In the system, before a converser A sends an enciphered message to a converser B, the converser A prepares an enciphering key (which represents a number obtained by calculating Y.sub.B.sbsp.A.sbsp.X (mod p)) generated from public information Y.sub.B of the converser B and secret information X.sub.A which is kept secret by the converser A. The number p is a large prime number of about 256 bits in binary representation, which is publicly known. a (mod b) means a remainder of division of the number a by the number b. The converser B also prepares the key wk in accordance to Y.sub.A.sbsp.B.sbsp.X (mod p) in a similar manner. Y.sub.A and Y.sub.B are selected so as to be equal to .alpha..sbsp.A.sbsp.X (mod p) and .alpha..sbsp.B.sbsp.X (mod p), respectively. As a result, Y.sub.B.sbsp.A.sbsp.X (mod p) becomes equal to Y.sub.A.sbsp.B.sbsp.X (mod p). It is known that even if Y.sub.A, .alpha. and p are known, it is infeasible for anybody except the converser A to obtain X.sub.A which satisfies Y.sub.A =.alpha..sbsp.A.sbsp.X (mod p).
The prior art key distribution system of the type described, however, has disadvantages in that since the system needs a large amount of public information corresponding to respective conversers, the amount of the public information increases as the number of conversers increases. Further, strict control of such information becomes necessary to prevent the information from being with tampered.